Swiss pilots planning to complete the first round-the-world solar flight next year unveiled a new version of their aircraft, which they say could remain in the air indefinitely.

solar Impulse
Solar Impulse II is seen in a hangar in Payerne, Switzerland. The new solar powered aircraft weights 2.4 tons with a wingspan of 72 metres and has more than 17,000 solar cellsReuters

At 72 metres (236 feet) its wingspan is eight metres longer than the first prototype, and even longer than that of a Boeing 747 jumbo jet. However, it weighs just 2.3 metric tons, about as much as a large car.

Bertrand Piccard and Andre Borschberg present their new solar-powered plane, the Solar Impulse HB-SIBAFP
Solar Impulse's wingspan is longer even than a Boeing 747 jumbo jet'sReuters

Bertrand Piccard and Andre Borschberg said the Solar Impulse 2 improves upon the single-seater prototype that first took flight five years ago, and which has since demonstrated that a solar-powered plane can fly through the night, between two continents and across the United States.

The materials are also lighter than before, it has more efficient electric motors, and a "good business class seat," Borschberg said. That's important, because while the journey will be broken up into several stages, the aircraft's slow speed means it will have to stay in the air for several days in a row during the long transoceanic legs.

Borschberg said the trip next year would take about 20 flying days, spread over several months.

2011 prototype
Ground staff are seen preparing a prototype of the Solar Impulse before a test flight at Payerne airport on April 18, 2011Reuters
2012 switzerland
The sun-powered Solar Impulse aircraft is pictured preparing for take-off on May 24, 2012 in Payerne on its first attempted intercontinental flight from Switzerland to Morocco with a stop-off in Madrid, without using a drop of fuelAFP
2012 morocco
Solar Impulse pilot Bertrand Piccard lands in Rabat, Morocco, after a 19-hour flight from Madrid on June 5, 2012Reuters
2013 wing
October 11, 2013: An engineer is pictured working on the 72-metre carbon-fibre wing of the newer Solar Impulse at a former military airport in Duebendorf, near ZurichAFP
breitling orbiter
In 1999, Bertrand Piccard and Brian Jones became the first people to fly non-stop round the world in a hot air balloon. The nine-ton Breitling Orbiter-3 took off from Switzerland and landed in Egypt nearly 20 days later after a 45,755 km (28,431 mile) flightReuters